The main motivation for this work is the need for a light source on silicon that is compatible with standard microelectronics processing. This need is driven by the limitations currently faced by the microelectronics industry due to the minimum physical sizes for copper interconnects—the main signal transmission system used on computer chips. As these interconnects are made smaller (currently down to a few tens of nm across) several unwanted problems arise, including RC time delay, heating or power dissipation, and electromagnetic interference (crosstalk). Currently, interconnects constitute about 70% of on-chip capacitance and approximately 1,200 m in total length per square cm of chip size. Unfortunately, as the number of transistors on a chip grows, so does the requirement for more interconnects, which also presents problems in terms of how to physically “pack in” all the required structures.
One hope to overcome the interconnect problem is to replace at least some of the electronic interconnects with optical ones. This would offer many advantages: light requires no physical transmission lines (although in some circumstances optical waveguides are expected to be required), optical signals generate no heat, no RC time delays, and light signals can “cross through” one another. However, despite these technical advantages, suitable light systems have not yet been found. The appropriate devices must include the three principal components of an optical communications system: the light source, the waveguide, and the detector. Our work is driven by the desire for a suitable light source.
A light source on silicon must meet several requirements to be used in integrated optics. First, it must be chemically compatible with silicon. That is, the materials comprising the light source must not adversely affect the rest of the chip. This has been a problem for III-V diodes made of materials such as GaAs. The reverse requirement is also true: it must be possible to grow the diode on a silicon wafer, which can lead to bonding or surface epitaxy problems. For these and other reasons, silicon itself may offer potential for a silicon-based LED, which would obviously circumvent the problems described above.
Traditionally silicon has proved to be a highly inefficient light emitter due to the indirect nature of the bandgap. This fact also means that the carrier lifetimes in silicon are long, which is undesired in optical communications in which the signals should be modulated rapidly. In recent years, however, there has been much development toward light-emitting silicon. Quantum confined silicon structures (e.g., porous silicon, or silicon nanocrystals) can emit light more efficiently than normal bulk silicon, with wavelengths typically in the visible part of the spectrum. Erbium-doped silicon and silicon nanostructures can act as light emitters in the 1.5 micron fiber transparency window, and LEDs have been demonstrated from these materials (M. Castagna, A. Muscara, S. Leonardi, S. Coffa, L. Caristia, C. Tringali and S. Lorenti, J. Lumin., 121, 187 (2006)). However, nanocrystals suffer from efficiency and conduction problems due to the oxide matrix in which they are embedded, porous silicon can be problematic for similar reasons and because of its fragility, the switching times are long, and erbium-doped silicon structures, although promising, have not yet proven sufficient for integration (due partly to low room-temperature intensities in the case of bulk silicon) and additional problems in the case of oxide films in which nanocrystals are used to sensitize the erbium fluorescence.
One promising materials system is bulk silicon itself. In the past 5-6 years, there have been several studies which have shown reasonably bright band-edge emission from silicon wafers (W. L. Ng, M. A. Lourenco, R. M. Gwilliam, S. Ledain, G. Shao, and K. P. Homewood, Nature 410, 192 (2001), M. A. Green, J. Zhao, A. Wang, P. J. Reese, and M. Gal, Nature 412, 805 (2001)). Much of this work has focused on p-n junctions, in which one of several methods can be employed to reduce the nonradiative recombination that has traditionally plagued bulk silicon. These methods generally involve some form of carrier confinement that prevents electrons and holes from sampling a large volume of the specimen where they might find nonradiative recombination pathways. Such confinement methods can involve the use of strain fields (D. J. Stowe, S. A. Galloway, S. Senkader, K. Mallik, R. J. Falster, and P. R. Wilshaw, Physica B 340-342, 710 (2003)), the concentration of electric fields at surface protuberances in the specimen (J. F. Harvey, R. A. Lux, and R. Tsu, Silicon nanostructure light-emitting diode. U.S. Pat. No. 5,627,386), or other physical methods in which the emitting region is made as small as possible (T. Hoang, P. LeMinh, J. Holleman, and J. Schmitz, IEEE Electron Device Letters, 28, 383 (2007) M. du Plessis, H. Aharoni, and L. W. Snyman, IEEE Photonics Tech. Lett. 14, 768 (2002)). In the case of tunnel diodes the carrier confinement arises as a result of band bending in a metal-oxide-semiconductor (MOS) device operated in accumulation mode (E.-Z. Liang, T.-W. Su, and C.-F. Lin, J. Appl. Phys. 100, 054509 (2006)).